Boats have used hydrofoils for many years as it is known that hydrofoils can reduce the drag of a hull going through the water and they can provide a smoother ride. There have been many variations of methods to control the ride height of the boat. All hydrofoil boats need some mechanism to make the boat fly at the right height above the water. Most of the mechanisms fall into one of two categories:                1. Incidence controlled has some mechanism that controls the angle of attack of the hydrofoil so that it keeps the boat at the right height.        2. Surface piercing foils control the lift generated by the foils by constantly varying the amount foil available in the water to provide lift. The foil pierces the surface of the water at an angle so that as the boat rises up there is less foil in the water and eventually the boat finds equilibrium.        
Surface piercing has the benefit of simplicity and potentially the ability to have the optimum amount of foil in the water, but incidence control has the following benefits:                1. The foil pierces the surface at an angle close to 90 degrees which produce less drag and spray and the foil is less susceptible to ventilation.        2. In rough water incidence controlled has more authority to lift the boat up and down quickly with the waves.        3. In the case of a sailboat the weather foil has the ability to pull down and keep the boat level regardless of the wind strength.        
Two examples of surface piercing are:                1. Sournat, et. al., U.S. Pat. No. 5,673,641.        2. Bernard Smith, U.S. Pat. No. 4,228,750.        
Ketterman, U.S. Pat. No. 5,168,824 and Clyde Jones hydrofoil catamaran U.S. Pat. No. 4,615,291 are both incidence controlled. In these two designs the strut and the hydrofoil are one solid foil and this whole foil pitches up and down to change angle of incidence, but the foil pivots about a point that is well above the foil and well above the water surface. This means that the lift vector and the drag vector generated by the hydrofoil are some distance away from the pivot and they create pitching moments on the hydrofoil. Also the predominate force, the lift vector, moves fore and aft as the hydrofoil pitches up and down which changes the pitching moments dramatically on the hydrofoil.
There are many designs that use a flap on the trailing edge of the hydrofoil to adjust lift which would be an example of an incidence controlled hydrofoil. This design is less efficient than a design where the whole hydrofoil pitches.
Incidence controlled hydrofoils typically have some type of sensor to sense the ride of the boat. The Ketterman patent hydrofoil uses a forward facing sensor which works well, but the sensors are well in front of the boat which are vulnerable and add considerable length to the boat.